WETLANDS - Water Quality Engineering in Natural Systems - page 294

8.3.1.3 Vertical Flow Wetlands. Vertical flow (VF)
wetlands are analogous to intermittent sand filters;
however, unlike sand filters, an outlet pipe provides air
into the lower reaches of the filter (see Fig. 8.8c),
thereby providing a greater capacity for oxidation. As
such, oxidation of ammonia and nitrification of waste-
waters can occur in VF wetlands. This makes VF wet-
lands particularly suitable for landfill leachates and
food processing wastewaters. Some variations of VF
wetlands do not provide for air entry into the lower
reaches of the system, inducing anaerobic conditions
that are conducive to immobilizing metals. Very con-
centrated wastewaters can be treated in VF systems,
and these systems can also be used for the dewatering
of sewage sludge.

Figure 8.10. Constructed subsurface flow wetland. Source :
Nova Scotia Agricultural College (2005).

8.3.2 Design of FWS Wetlands

Design goals for FWS wetlands range from an exclusive
commitment to treatment functions to systems that
provide advanced treatment combined with enhanced
wildlife habitat and public recreational opportunities.
The size of constructed wetlands range from small on-
site units to large systems covering over 15,000 ha
(37,000 ac) that are used as treatment areas for agricul-
tural runoff. The design of FWS wetlands are either
technology-based or performance-based. Technology-
based approaches simply use regulatory prescribed
loading rates to size the wetland, while performance-
based approaches use targeted outflow concentrations,
targeted effluent loads, and/or targeted removal rates as
bases for sizing the wetland. Small onsite wetlands and
urban stormwater wetlands typically use the technology-
based approach, while large systems typically use the
performance-based approach.

domestic wastewater treatment after primary settling
with good success in meeting secondary effluent stan-
dards (TSS, BOD ≤ 30 mg/L).
HSSF wetlands contain soil, gravel, or rock media
with emergent aquatic plants rooted in the media. In the
United States, the most common medium is gravel, but
sand and soil have been used in Europe. Most con-
trolled studies indicate that these systems perform as
well without plants as with plants (Kreissl and Brown,
2000). In operational systems in the United States, the
porous medium depth is typically in the range of 30-
90 cm (1-3 ft), and design flows range from less than
190 m 3 /day (50,000 gpd) to 13,000 m 3 /day (3.5 mgd).
The HSSF system is underlain with an impermeable
layer of clay or synthetic liner, and the system is built
with a slight inclination of 1-3% between the inlet and
the outlet. A typical HSSF wetland is shown in Figure
8.10, where the surface vegetation is in the early stage
of development. Inlet piping, berms, and outlet piping
with water level control are ancillary features of HSSF
wetlands.
Problems have arisen when HSSF systems were used
to remove nitrogen or perform other functions for
which they are not physically capable. In both FWS and
HSSF wetlands, physical pollutant removal may be tem-
porary because chemical and microbiological degrada-
tion in anaerobic areas can return some settled organic
and inorganic matter to the water column by transform-
ing particulate matter to other, more soluble, matter.
HSSF wetlands have been used extensively in Europe
under the names root zone method, hydrobotanical
system, soil filter trench, biological macrophytic and
marsh bed, and vegetated submerged bed.

8.3.2.1 Hydrology and Hydraulics. Hydrology is the
most important variable in wetland design. If the proper
hydrologic conditions are developed, chemical and bio-
logical conditions will respond accordingly. Parameters
used to characterize the hydrologic conditions of treat-
ment wetlands include hydroperiod, seasonal pulses,
hydraulic loading rate, and detention time. For large
wetlands, the volume of water leaving the wetland is
rarely the same as the volume entering the wetland,
since there are gains due to precipitation and losses due
to ET and sometimes infiltration.

Hydroperiod. The hydroperiod is defined as the pattern
of water depth fluctuation over time. Wetlands that have
a seasonal fluctuation of water depth have the most
potential for developing a diversity of plants, animals,
and biogeochemical processes. Alternate flooding and
aeration of soils promote nitrification-denitrification,